Because for some they are really hard to come up with and you really have to have wit to come up with an original.
Also, since it insults more than one person. One, the person who you directly insulted, because they came out of their mom. Two, the mom, for obvious reasons. Three, the dad, for marrying the mom. Four, the siblings because they came outta there too. Fifth, the grandparents for giving birth to the ma....I can go all night.